Prestressed concrete



Dec. 14, 1965 c. FRANCIS 3,222,835

PRESTRESSED CONCRETE Filed May 29, 1961 ROTATION INVENTOR. Gerald C. Frauds United States Patent 3,222,835 PRESTRESSED CONCRETE Gerald C. Francis, P.0. Box 512, Lansing, Mich. Filed May 29, 1961, Ser. No. 113,457 4 Claims. (Cl. 52223) My invention relates to prestressed concrete and more particularly to a prestressed slab form of concrete and a method of prestressing same.

Various problems have been encountered in the prestressing of concrete slabs, particularly with regard to providing substantially uniform stresses. Other problems are in the setting up and operation of the mechanisms used to provide the prestressing.

An object of the present invention is to simplify concrete prestressing by providing an improved method of setting up and operating prestressing mechanisms.

Another object of the present invention is to facilitate prestressing of a concrete slab by setting up spaced frame members connected with cables, moving one frame mem-- ber relative to another to induce stress in the cables, pouring concrete between the frame members and around the cables, then releasing the frame members whereby cable stresses are imparted to the concrete.

A further object of the invention is to contruct a new type of prestressed concrete slab by producing unique stresses between a central area and the periphery of the slab.

For a more complete understanding of the invention, reference may be had to the accompanying drawing illustrating some preferred embodiments of the invention in which like reference characters refer to like parts throughout the several views and in which:

FIG. 1 is a plan elevational view of a preferred structure in stressed position preparatory to pouring concrete.

FIG. 2 is a cross-sectional view taken substantially on the line 22 of FIG. 1 but including the concrete as in the completed slab.

FIG. 3 is a plan elevational view of another preferred structure as initially provided before stressing.

FIGS. 1 and 2 illustrate the structure and method of fabricating a circular slab, while FIG. 3 illustrates the structure and method of fabricating a polygonal slab.

The circular structure is set up by providing an inner tension frame member and an outer compression frame member 11 spaced from the inner member 10. A plurality of cables 12, all of equal length, connect the members 10 and 11 and are secured initially with no stress on radial lines indicated by the dash lines in FIG. 1.

The frame members 10 and 11 preferably are T-shaped as shown, having respectively outwardly extending and inwardly extending feet or web flanges 10A and 11A, and peripheral head elements 10B and 11B respectively.

The cables 12 are provided at each end with connector fittings such as clevises 13 secured to equally spaced points on the flanges 10A and 11A by any means such as rivets 14 or the like.

One of the frame members, preferably the inner member 10, is secured to and immobilized by a mounting structure 15 preferably secured to a foundation structure 16.

Next, the outer frame 11 is rotated as indicated by the arcuate arrow to the full line position of FIG. 1. This increases the distance between each cables connecting points without altering the general radial distance between the frame members 10 and 11. This induces tension stresses in the inner frame member 10 and the cables 12 while inducing compression stresses in the outer frame member 11.

The outer frame member is then immobilized or fixed in this position by any preferred means (not shown). It

3,222,835 Patented Dec. 14, 1965 will be seen that the cables 12 have assumed a position such that they diverge angularly from their initial radial lines, and inwardly projecting lines from the cables will circumscribe a circle, as indicated in FIG. 1, concentric with the common center point or axis A of the frame members 10 and 11.

Once fixed, a form 20 is preferably constructed as indicated in FIG. 2 on the lower side of the assembly. A concrete slab 21 is then poured between the frame members 10 and 11, enclosing the cables 12, and the connecting elements.

After the concrete has set, the form 20 is removed and the frame members 10 and 11 are released. The stresses in the cables are thereby imparted to the concrete in the well-known manner, producing a ready-made slab of uniformly prestressed concrete provided with securely retained inner and outer peripheral rims.

This complete integral assembly may be made in many sizes. It is contemplated for use both as an airplane hanger floor and as a table-top. It will be apparent that the number of cables used may be greater or lesser than the number shown, depending on the size and required prestressing.

FIG. 3 illustrates the structure in which the present method is used to produce a non circular slab. For convenience, a square form is shown as comprising a circular inner tension frame member 25 and a square outer compression frame member 26 preferably made of T-section as the members in FIGS. 1 and 2 connected by cables 27 and 28.

The outer member 26 is secured and held in shape pref erably by a truss assembly made substantial enough to prevent a tendency of the outer member 26 to deform when it is rotated to the dash-line position after the inner member 25 is immobilized.

It is noted that the cables 27 and 28 are of unequal length, and thus the changes in dimension produced between their respective connecting points on rotation will vary, there being a greater change in the shorter than the longer dimensions. Thus it may be necessary to providea greater initial degree of slack in the shorter cables 27, determined with reference to the degree of rotation required to produce the desired uniform tension in all cables.

Other than the above, the method of construction, applying tension, pouring the concrete, and releasing the frame members to transmit stresses to the concrete, will be substantially the same as the method described for the structure of FIGS. 1 and 2.

It will be noted that either the inner or outer frame member could be rotated, but it is easier, due to the greater leverage, to rotate the outer.

The net result of the described methods is seen to provide a relatively simplified system for producing substantially uniform prestressing of concrete slabs which, when completed, are ready-made with integral rims and have a central open space which may have different uses. When used for a hanger floor or roof, for example, the central area may be used for a center post. When used as a table top, the hole is a convenient location for a planter or the like.

Although I have described only two embodiments of the invention, it will be apparent to one skilled in the art to which the invention pertains that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. A method of prestressing a concrete slab, comprising: locating an inner tension member and a peripheral compression member on a common central axis with the compression member radially outwardly spaced from the tension member, connecting said members with a plurality of cables extending in annularly spaced radial lines, immobilizing one of said members, rotating the other of said members about said common center whereby to impart tension loading to said cables, immobilizing said rotated member, pouring a slab of concrete between said members and around said cables, allowing the concrete to set and then releasing said members whereby cable stresses are imparted to said concrete.

2. A method of prestressing a concrete section, comprising: spacing a tension member and a compression member at an established distance one from the other and on a common axis, connecting a cable from a point on one member to a point on the other member, rotating one member with respect to the other about the common axis to increase the distance between the cable connecting points whereby to apply tension to the cable Without altering the spatial distance between said members, affixing said members in the rotated position, pouring concrete between said members and around said cable, allowing the concrete to set and then releasing said rotated member whereby to transmit the cable stress to said concrete.

3. A method of prestressing a concrete section, comprising: spacing an inner circular tension member and a larger outer circular compression member concentrically with respect to each other, connecting said members with a plurality of annularly spaced radially extending cables, immobilizing one of said members, rotating the other of said members while retaining the aforesaid concentricity to impart tension loading to said cables, immobilizing said rotated member, pouring concrete between said members and around said cables, allowing the concrete to set and then releasing said members whereby cable stresses are imparted to said concrete.

4. A method of prestressing a concrete section, comprising: spacing an inner circular tension member and a larger outer polygonal compression member on a common central axis, with the compression member radially outwardly spaced from the tension member, attaching a polygonal truss structure to said compression member to retain the rigidity thereof, connecting said members with a plurality of annularly spaced radially extending cables, immobilizing one of said members, rotating the other of said members about said common axis whereby to impart tension loading to said cables, immobilizing said rotated member, pouring concrete between said members and around said cables, allowing the concrete to set and then releasing said members and detaching said truss structure whereby cable stresses are imparted to said concrete.

References Cited by the Examiner UNITED STATES PATENTS 584,068 6/1897 Weber 52-82 1,193,767 8/1916 Crisell 52-82 1,559,837 11/1925 Allen 52-225 2,411,651 11/1946 Darby 52-82 2,609,586 9/1952 Parry 25-154 2,692,566 10/1954 Mitchell 52-63 2,850,892 9/1958 Stump 52-223 2,859,504 11/1958 Crowley 25-154 2,941,394 6/1960 Brandt 52-223 FOREIGN PATENTS 1,251,651 12/1960 France.

JACOB L. NACKENOFF, Primary Examiner.

WILLIAM I. MUSHAKE, BENJAMIN BENDETT.

HENRY C. SUTHERLAND, Examiners. 

1. A METHOD OF PRESTRESSING A CONCRETE SLAB, COMPRISING: LOCATING AN INNER TENSION MEMBER AND A PERIPHERAL COMPRESSION MEMBER ON A COMMON CENTRAL AXIS WITH THE COMPRESSION MEMBER RADIALLY OUTWARDLY SPACED FROM THE TENSION MEMBER, CONNECTING SAID MEMBERS WITH A PLURALITY OF CABLES EXTENDING IN ANNULARLY SPACED RADIAL LINES, IMMOBILIZING ONE OF SAID MEMBERS, ROTATING THE OTHER OF SAID MEMBERS ABOUT SAID COMMON CENTER WHEREBY TO IMPART TENSION LOADING TO SAID CABLES, IMMOBILIZING SAID ROTATED MEMBER, POURING A SLAB OF CONCRETE BETWEEN SAID MEMBERS AND AROUND SAID CABLES, ALLOWING THE CONCRETE TO SET AND THEN RELEASING SAID MEMBERS WHEREBY CABLE STRESSES ARE IMPARTED TO SAID CONCRETE. 